1. Field of the Invention
The present invention relates generally to methods of forming a metal silicate film used in a semiconductor device, and more particularly to a method of manufacturing a minute high-speed semiconductor device having a high-dielectric-constant film.
2. Description of the Related Art
In today's very high speed semiconductor devices, it is becoming possible to achieve a gate length of 0.1 μm or less with progress in miniaturization processes. In general, the more miniaturized a semiconductor device, the higher its operating speed. In such very minute semiconductor devices, however, a decrease in the gate length due to miniaturization requires the film thickness of a gate insulating film to be reduced in accordance with a scaling law.
However, the gate length of 0.1 μm or less requires the gate insulating film to be 1-2 nm or less in thickness if a conventional thermal oxide film is used for the gate insulating film. Such a very thin gate insulating film causes tunneling current to increase, and the problem of a resulting increase in gate leakage current is inevitable.
Such being the case, it has been proposed to apply high-dielectric-constant materials (so-called high-K materials) to the gate insulating film, such as Ta2O5, Al2O3, ZrO2, HfO2, ZrSiOx, and HfSiOx, which have much higher dielectric constants than the thermal oxide film. Employment of such high-dielectric-constant materials makes it possible to increase physical film thickness while keeping EOT (Equivalent Oxide Thickness) small (EOT shows the thickness of a SiO2 film having the same specific capacitance as the high-dielectric-constant film in question). Accordingly, even in a very high speed semiconductor device having an extremely small gate length of 0.1 μm or less, it is possible to employ a gate insulating film of approximately 10 nm in physical film thickness, so that it is possible to suppress gate leakage current due to the tunnel effect.
In particular, metal silicate materials such as ZrSiOx and HfSiOx have significantly high crystallization temperatures compared with oxide materials such as ZrO2 and HfO2 although their dielectric constants are slightly lower than those of the oxide materials. Accordingly, it is possible to control occurrence of crystallization in a film effectively even in the case of performing heat treatment employed in a semiconductor device manufacturing process. Therefore, the metal silicate materials are considered extremely suitable for the high-dielectric-constant gate insulating film of the high-speed semiconductor device. Further, it is conventionally performed to introduce nitrogen into a high-dielectric-constant film in order to control crystallization and to prevent diffusion of a dopant having a high diffusion coefficient, such as B, in particular, from a gate electrode into a channel region. In the case of causing an increase in crystallization temperature by introducing nitrogen, it is preferable that nitrogen atoms be bonded to Si atoms in the high-dielectric-constant gate insulating film, which is accordingly required to contain a sufficient amount of Si atoms.
Conventionally, it is known that such a high-dielectric-constant gate insulating film can be formed by Atomic Layer Deposition (ALD) or metalorganic CVD (MOCVD). In particular, in the case of employing ALD, by which a film is formed by depositing atomic layers one by one, it is possible to form any composition gradient in the film. On the other hand, however, ALD is time-consuming because the material gas is switched every atomic layer and a purging process is performed between depositions of atomic layers. Accordingly, ALD has the problem of reduced throughput of manufacturing semiconductor devices.
On the other hand, according to MOCVD, deposition is performed one time using an organometallic compound material. Accordingly, it is possible to achieve significant improvement of the throughput of manufacturing semiconductor devices. Therefore, it is preferred to employ MOCVD rather than ALD in order to improve productivity. Further, a film formation apparatus using MOCVD is simpler in structure than a film formation apparatus using ALD. Accordingly, the MOCVD apparatus enjoys the advantage of reduced apparatus cost and reduced apparatus maintenance cost compared with the ALD apparatus.
Next, a description is given below of a high-speed semiconductor device including a high-dielectric-constant film formed by MOCVD. FIG. 1 is a schematic diagram showing a high-speed semiconductor device 10 having a high-dielectric-constant gate insulating film.
Referring to FIG. 1, the semiconductor device 10 is formed on a silicon substrate 11. A high-dielectric-constant gate insulating film 13 such as HfSiOx is formed on the silicon substrate 11 with a thin base oxide film 12 provided therebetween. Further, a gate electrode 14 is formed on the high-dielectric-constant gate insulating film 13.
According to the semiconductor device 10 of FIG. 1, an oxynitride film 12A is formed by doping the surface part of the base oxide film 12 with such a level of nitrogen (N) as to maintain the flatness of the interface between the silicon substrate 11 and the base oxide film 12. The EOT of the base oxide film 12 is further reduced by forming the oxynitride film 12A having a higher dielectric constant than a silicon oxide film in the base oxide film 12.
As described above, in actual semiconductor devices, an extremely thin base oxide film (silicon oxide film, SiO2) is provided between a high-dielectric-constant insulating film and a silicon substrate in order to improve carrier mobility in a channel region. In using the base oxide film, nitrogen is added thereto if necessary.
In this case, the base oxide film is required to be extremely thin. If the base oxide film is thick, the effective EOT increases even in combination with the high-dielectric-constant insulating film formed thereon, thus canceling the effect produced by using the high-dielectric-constant insulating film for a gate insulating film. Meanwhile, such an extremely thin base oxide film is required to evenly cover the surface of the silicon substrate and not to form a defect such as an interface state.
For example, it has been proposed to form the above-described base oxide film after forming a metal silicate film in the case of forming the metal silicate film as a high-dielectric-constant gate insulating film by MOCVD using an amidic organic material (see, for example, PCT International Application Publication No. WO 03/088341).
For example, in the case of forming a metal silicate film by MOCVD using an amidic organic material, it is preferable to provide the process of removing excess carbon (C) in the metal silicate film by performing plasma processing after forming the metal silicate film. Therefore, there has been proposed the method of forming a base oxide film by oxidizing the surface of a silicon substrate through the metal silicate film using plasma processing.
Even in the case of forming a base oxide film by the above-described method, however, there is a problem in that the effects of the film quality and film thickness of a film formed in the beginning of film formation by MOCVD using a high-dielectric-constant material are not taken into consideration.
As described above, the base oxide film is very thin, and preferably, is formed to be, for example, 0.8 nm or less in thickness. Therefore, there may be a case where the film quality of a film in an extremely thin region formed in the beginning of film formation in forming a metal silicate film by MOCVD becomes important. Conventionally, little consideration is given of such film quality and film thickness in the beginning of film formation. In the case of MOCVD in particular, there may be a difference between the film quality of an extremely thin region formed in the beginning of film formation and the film quality at the later stage in the film formation. If the film quality thus changes in the vicinity of the interface between the base oxide film and the metal silicate film, there is a problem in that film qualities such as a dielectric constant and EOT deviate from desired values.
For example, even if the base oxide film is formed to be very thin with excellent film quality, because of the effects of changes in the film quality of a high-dielectric-constant silicate film in the beginning of its formation by MOCVD, desired electrical characteristics may not be obtained when a device is formed.
In particular, in the case of using an amidic material gas containing no oxygen in its material composition, the degree of oxidation of the material gas differs depending on the state of feeding of the material gas and an oxidation gas onto the silicon substrate. Accordingly, the film quality may differ greatly, and there is a problem in that there is great variation in the film quality in the beginning of film formation in particular.